VirtualBox free vitualization software fox x86 computers
VirtualBox is a powerfull open source virtual machine for x86 based systems, (which you can run windows on linux or vice versa)
It runs on most Mac Os X Windows Linux Solaris and OpenBSD.
VirtualBox supports many guest operating systems, here is the list :
http://www.virtualbox.org/wiki/Guest_OSes
I have tried it installing Pardus Linux that is not listed there and it worked well, (better than qemu)
I have also tried to install Leopard (Hackintosh) on it but i could only boot and it did not run further.